950 



SPERM, OVA, AND PREGNANCY 



the rate of transfer of simple substances by 

 diffusion across the barrier increases, the 

 rate of absorption of more complex sub- 

 stances by pinocytosis declines. In rodents' 

 placentas, on the other hand, although the 

 trophoblastic cells of the placental laby- 

 rinth become thinner and their cytoplasm 

 more simplified, pinocytosis seems to persist 

 throughout gestation in the epithelium of 

 the yolk sac thereby affording a continuous 

 means for the absorption of substances of 

 larger molecular size. Comparative studies 

 of the placentas of animals with the elec- 

 tron microscope are just beginning, so that, 

 although it seems that pinocytosis might 

 play an important role in placental phys- 

 iologic exchange, no broader generalizations 

 regarding its significance can at present be 

 offered. It is apparent that in the future more 

 attention will have to be paid to the chem- 

 ical, histochemical, and cytologic structure 

 of every part of the barrier with respect to 

 the mode of passage of different kinds of 

 substances. JMoreover, a comprehensive the- 

 ory of placental exchange, from the com- 

 parative and phylogenic point of view, will 

 have to take cognizance of all of the struc- 

 tures forming the placental barrier, instead 

 of confining itself to the chorio-allantoic pla- 

 centa as defined in terms of Grosser's four 

 placental types. 



XII. References 



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